Sugar vanadates: synthesis and characterisation of mannopyranoside and ribofuranoside esters incorporating VO~(3+)

摘要

The sugars, methyl 4, 6-di-O-methyl-#alpha#-D-mannopyranoside (H_2m) and methyl 5-O-methyl-#beta#-D-ribofuranoside (H_2r) have been synthesized. These react smoothly with [V~(IV)O(L)(H_2O)] in methanol in air affording the dark coloured vanadates [V~VO(Hm)(L)] and [V~VO(Hr)(L)] in excellent yields (L~(2-) = N-salicylideneglycinate). The crystal structure determination of [V~VO(Hm)(L)] revealed five-membered chelation of VO~(3+) by the alcoholic and alkoxide oxygen atoms of the monoionised carbohydrate. The two atoms lie respectively trans to the oxo oxygen and aldine nitrogen atoms. The five V-O bonds are unequal and span the range 1.57-2.37 A. The asymmetric unit of the complex consists of two metrically similar molecules locked in carboxylate-alcohol O … O hydrogen bonding generating a macrocyclic cavity. In solution each ester displays a single ~(51)V resonance near #delta# -544. The ~1H NMR parameters of the alkoxidic and alcoholic CH protons and of OMe protons are consistent with the O, O-chelation mode for both the vanadates.